That’s a tricky case. Giovanni Antonio Boltraffio worked in the studio of Leonardo in the period in which he was in Milan. The attribution of this Madonna With Child (or Madonna Litta) is disputed among Leonardo, Boltraffio and Marco d’Oggiono. Art critics generally say that the painting was actually mostly painted and completed by Boltraffio (possibly on ideas and primary sketches by Da Vinci). Obviously a museum attaches no doubt on Leonardo’s autorship because Leonardo is known all over the world, whereas the name of Giovanni Antonio Boltraffio is known only to the insiders.http://en.wikipedia.org/wiki/Giovanni_Antonio_Boltraffio